The present invention relates to a synchronous rectifying control circuit of a flyback switching power supply and, more particularly, to a control circuit used on a secondary side circuit of a flyback switching power supply and capable of synchronously controlling a plurality of sets of rectifying outputs and synchronously adjusting the waveforms of rectifying outputs.
In existent DC power supply devices like AC to DC switching power supplies, in order to shrink the volume, a high-frequency pulse width modulation (PWM) controller is exploited to control the DC output voltage. As shown in FIG. 1, the front-stage circuit and the rear-stage circuit of a transformer T1 are divided into a primary side 101 and a secondary side 102. A phototransistor 111 and a photodiode 112 between the primary side 101 and the secondary side 102 are used to separate electric signals of the primary side 101 and the secondary side 102. Optical signals can be used to feed back the voltage or current output variation signal of the secondary side 102 to the primary side 101 for synchronous adjustment of the voltage and current variation of the primary side 101 and the secondary side 102, or can be used as feedback signals for over-current and short-circuit protection.
In the prior art, a flyback power supply is primarily a circuit for converting an AC voltage into a DC voltage. It is based on the following principle. When a field-effect transistor (FET) Q1 on the primary side 101 conducts, energy cannot be transferred through a rectifying diode D1 due to polarity inversion of the primary side and the secondary side, and is stored on the transformer T1. After the conduction period of the FET Q1 is over, the polarity of the transformer T1 inverses, and the rectifying diode D1 on the secondary side conducts to release energy stored on the transformer T1. However, when the rectifying diode D1 conducts, there is a voltage drop of about 0.4-1.5V according to the magnitude of the output load. This voltage drop multiplied by the load current is the dissipation power loss of the rectifying diode D1. If the load current is large, the output power source efficiency of the DC power supply device is much reduced.
For an existent flyback switching power supply, there generally is more than one set of output power sources. If all the output power sources are to have synchronous output voltages and waveforms, a very complicated circuit is required. Therefore, in order to save costs, most of existent flyback switching power supplies have no synchronous output function, and the output voltages and waveforms thereof need to be adjusted separately.
The primary object of the present invention is to provide a synchronous rectifying control circuit of a flyback switching power supply, which is connected at a secondary side of a transformer and has a plurality of sets of power source ends and a set of induction ends. The power source ends comprise a first power source output end and a secondary power source output end. The set of induction ends comprises a first induction end and a second induction end. The secondary power source output end of the power source ends is connected to a rectifying circuit for rectifying the voltage waveform between the first power source output end of the power source ends and a reference potential end. The first induction end of the set of induction ends is connected to a rectifying diode, which is used to rectify the first induction end and the reference potential end to form a detection end having a waveform with the same phase as the power source ends. A synchronous control circuit is connected to the first power source output and the detection end, and comprises a synchronous input end, a control end and a waveform adjustment end. The synchronous input end is used for input of a synchronous signal. The control end is connected to the rectifying circuit and used to synchronously control the rectifying circuit to generate a uniform rectifying response period, hence accomplishing synchronous adjustment of several sets of output voltage waveforms.
Another object of the present invention is to provide a synchronous rectifying control circuit of a flyback switching power supply, wherein a comparator is provided. The comparator has a positive input end, a negative input end and an output end. The positive input end connects a first resistor to the detection end and connects a second resistor to the reference potential end. The negative input end connects a third resistor to the first power source output end and connects a fourth resistor to the reference potential end. The output end connects a fifth resistor for feedback to the positive input end to convert the waveform of the detection end into a waveform having an apparent slope, hence accomplishing synchronous adjustment of the control circuit to increase or decrease the response period of the rectifying circuit.
Yet another object of the present invention is to provide a synchronous rectifying control circuit of a flyback switching power supply, which is connected at a secondary side of a transformer and has a plurality of sets of power source ends and a set of induction ends. The power source ends comprise a first power source output end and a secondary power source output end. The set of induction ends comprises a first induction end and a second induction end. The drain (D) of an FET is connected to the second power source output end and the source (S) of the FET is connected to a reference potential end so that the voltage waveform between the first power source output end and the reference potential end can be rectified through conducting or shutting off the FET. The first induction end is connected to a rectifying diode to rectify the first induction end and the reference potential end to form a detection end having a waveform with the same phase as the first power source output end. A comparator is used to convert the waveform at the detection end into a waveform having an apparent slope. A drive control circuit has a control input end and a control output end. The control input end is connected to the output end of the comparator and the control output end is connected to a gate (G) of the FET to drive and control the FET for increasing or decreasing the rectifying response period.